The present invention relates to a hydraulic power shovel such as loading shovel or a back hoe shovel, equipped with a swingable boom, an arm pivotally connected to the boom and a bucket held by the arm, wherein particularly, the invention is concerned with a hydraulic power shovel provided with an automatic horizontal driving system for horizontally driving the bucket.
In digging operation in a loading shovel, it is necessary to drive the bucket horizontally and forwardly by swinging the arm upwardly while swinging the boom downwardly. It is, however, extremely difficult to simultaneously manipulate both of the arm cylinder for driving the arm and the boom cylinder for driving the boom.
In order to eliminate this difficulty, in, for example, U.S. Pat. No. 3,927,781, an automatic horizontal driving system has been developed in which the boom is automatically driven pivotally in response to the pivotal movement of the arm so that the bucket moves horizontally solely through the manipulation of the arm cylinder.
This known horizontal driving system is constituted by a levelling cylinder adapted to extend and contract in accordance with the movement of the arm, and a hydraulic circuit for hydraulically connecting the levelling cylinder to the boom cylinder for driving the boom.
In operation, after positioning the arm such that the bucket attached to the end thereof contacts the ground, the arm is pivotally moved forwardly. This pivotal movement of the arm causes the levelling cylinder to extend so that the working oil in the rod-side chamber in the levelling cylinder is delivered to the rod-side chamber of the boom cylinder and the working oil in the bottom-side chamber of the boom cylinder is drawn into the bottom-side chamber of the levelling cylinder to retract the boom cylinder to thereby swing the boom downwardly. Consequently, the bucket is driven horizontally and forwardly along the ground surface to perform the digging operation. This known horizontal driving system, however, has encountered problems or drawbacks due to the fact that the levelling cylinder is continuously held in fluid connection with the boom cylinder, as will be realized from the following description.
One cycle of loading operation includes the steps of conducting the digging by pushing the bucket horizontally and forwardly through forward swinging of the arm, swinging the boom upwardly to raise the bucket to release the soil and sand from the bucket to the bed of a dump truck, swinging the arm downwardly and then swinging the boom downwardly to return the bucket to the starting position. When the arm is swung downwardly, the working oil is forced out from the levelling cylinder into the boom cylinder, so that the boom is swung upwardly. Consequently, the required downward stroke of swinging of the boom is undesirably increased. When the arm is swung downwardly while the boom has been swung to the upper limit position, the boom cylinder cannot extend further even though the working oil is forcibly delivered from the bottom-side chamber of the levelling cylinder to the bottom-side chamber of the boom cylinder. Consequently, the working oil is confined in the levelling cylinder so that the oil pressure is increased in the bottom-side chambers of the levelling cylinder and the boom cylinder. As this oil pressure is increased to the set pressure of a relief valve connected to the bottom-side chamber of the boom cylinder, the relief valve is opened to release the oil from the bottom-side chamber of the levelling cylinder, so that the levelling cylinder is allowed to contract to permit the arm to be swung downwardly. In such a case, therefore, an impractically large energy is required for contracting the levelling cylinder, i.e. for swinging the arm downwardly.
In some sites, it is required to move the bucket along an arcuate path from the lower side to the upper side insteadly of pushing the bucket forwardly and horizontally. The conventional horizontal driving system, however, cannot cope with such a demand.